Diabetes is a multi-cause metabolic disease, which is characterized by chronic high blood glucose, accompanied by sugar, fat and protein metabolism disorder caused by the defect of insulin secretion and/or action. Diabetes is also a very old disease, which is due to the relative or absolute lack of insulin in the human body, causing the increased concentration of blood glucose, resulting in sugar discharged in a large amount with the urine, and accompanied by polydipsia, polyuria, polyphagia, weight loss, dizziness, fatigue and other symptoms.
In the treatment of diabetes, exercise therapy and diet therapy are two essential treatments of diabetes. When these two treatments are not sufficient to control the disease condition, insulin or oral hypoglycemic agents can be used. However, since these existing hypoglycemic agents have too many side effects, it is particularly important to develop a novel drug with less side effects and more therapeutic effects in the treatment of diabetes.
Dipeptidyl peptidase IV (DPP-IV) is a serine protease, which can selectively cleave the N-terminus dipeptide of the peptide chain containing one proline residue at the penultimate position from the N-terminal. Although the physiological effect of DPP-IV on mammals has not been fully confirmed, it plays an important role in neuropeptide metabolism, T-cell activation, adhesion of cancer cells and endothelium, process of HIV virus coming into lymphocytes, and other processes (see WO 98/19998).
Studies have shown that, DPP-IV can degrade glucagon-like peptide (GLP-1), i.e., by cleaving the histidine-alanine dipeptide at N-terminal of GLP-1, the GLP-1 in its active form can be degraded into inactive GLP-1-(7-36) amide, which is further degraded into inactive GLP-1-(9-36) amide (see, Hansen L, Deacon C F, Ørskov C, et al., Endocrinology, 1999, 140: 5356-5363). In physiological conditions, the half-life of intact GLP-1 in circulatory blood is very short, and the inactive metabolites obtained from the degradation of GLP-1 by DPP-IV can bind with the GLP-1 receptor to antagonize the active GLP-1, so as to shorten the physiological responses of GLP 1 receptor to GLP-1, while the DPP-IV inhibitors can completely protect endogenous, even exogenous GLP-1 from being inactivated by DPP-IV, and thus greatly increase the physiology activity of GLP-1 (by 5-10 times). GLP-1 is an important stimulus to the secretion of pancreatic insulin and can directly influence the distribution of glucose, therefore, DPP-IV inhibitors play a very positive role in the treatment of patients with non-insulin-dependent diabetes (U.S. Pat. No. 6,110,949).